On March 11, 2024 5:53:33 PM PDT, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: >On 3/11/24 16:56, Nadav Amit wrote: >> So you can look on the dirty-bit, which is not being set >> speculatively and save yourself one problem. >Define "set speculatively". :) > >> If software on one logical processor writes to a page while software >> on another logical processor concurrently clears the R/W flag in the >> paging-structure entry that maps the page, execution on some >> processors may result in the entry’s dirty flag being set (due to the >> write on the first logical processor) and the entry’s R/W flag being >> clear (due to the update to the entry on the second logical >> processor). > >In other words, you'll see both a fault *AND* the dirty bit. The write >never retired and the dirty bit is set. > >Does that count as being set speculatively? > >That's just the behavior that the SDM explicitly admits to. Indeed; both the A and D bits are by design permissive; that is, the hardware can set them at any time. The only guarantees are: 1. The hardware will not set the A bit on a not present late, nor the D bit on a read only page. 2. *Provided that the user has invalidated the page entry in the TLB*, hardware guarantees the respective bits will be set before a dependent memory access is made visible. Thus the bits are guaranteed to reflect a strict superset of operations performed architecturally.
